Parents will no longer get tax credit for ‘child no.3+’ born on or after 6th of April 2017. This applies to children born after child ‘no 2’ on or after the date. But there are exceptions for this rule, according to HM Government
The changes does not affect children born before 6th of April or 1st and 2nd born child.
Full details of exception are available on HMRC website.